Chapter 5

Question 1

. However, most protein-ligand interactions do
not involve a prosthetic group. Instead, the binding site for
a ligand is more o

Answer 1

cleftin the protein lined with amino acid residues, arranged to
make the binding interaction highly specific. c. Effective
discrimination between ligands is the norm at binding sites, even
when the ligands have only minor structural differences.

Question 2

humoral immune system

Answer 2

The immune response consists of two complementary systems,
the humoral and cellular immune systems. The humoral
immune system (Latin humor,
“fluid”) is directed at bacterial
infections and extracellular viruses (those found in the body
fluids), but it can also respond to individual foreign proteins.

Question 3

cellular immune system

Answer 3

The
cellular immune system destroys host cells infected by viruses
and also destroys some parasites and foreign tissues.

Question 4

immunity- p683, read before test maybe

Answer 4

kk

Question 5

antigenic determinant or epitope.

Answer 5

An individual antibody or T-cell receptor
binds only a particular molecular structure within the antigen,
called its

Question 6

Immunoglobulin G (IgG)

Answer 6

) is the major class of antibody molecule
and one of the most abundant proteins in the blood serum. IgG
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has four polypeptide chains: two large ones, called heavy chains,
and two smaller ones, called light chains, linked by noncovalent
and disulfide bonds into a complex of Mr 150,000.

Question 7

The heavy
chains of an IgG molecule interact at one end, then branch to
interact separately with the light chains, forming a Y-shaped
molecule (Fig. 5-20). At the “hinges” separating the base of an IgG
molecule from its branches, good pic on p 686 btw

Answer 7

the immunoglobulin can be cleaved
with proteases. Cleavage with the protease papain liberates the
basal fragment, called Fc because it usually crystallizes readily,
and the two branches, called Fab, the antigen-binding fragments.
Each branch has a single antigen-binding site.

Question 8

In many vertebrates, IgG is but one of

Answer 8

f five classes of
immunoglobulins. Each class has a characteristic type of heavy
chain, denoted α, δ, ε, γ, and μ for IgA, IgD, IgE, IgG, and
IgM, respectively

Question 9

The immune response is mediated by

Answer 9

interactions among an
array of specialized leukocytes and their associated proteins. T
lymphocytes produce T-cell receptors. B lymphocytes produce
immunoglobulins.

Question 10

Humans have five classes of immunoglobulins, each with
different biological functions. The most abundant class i

Answer 10

s is IgG, a
Y-shaped protein with two heavy chains and two light chains. The
domains near the upper ends of the Y are hypervariable within
the broad population of IgGs and form two antigen-binding sites.

Question 11

A given immunoglobulin generally binds to only a

Answer 11

part, called
the epitope, of a large antigen. Binding often n involves a
conformational change in the IgG, an induced fit to the antigen.

Question 12

The exquisite binding specificity of immunoglobulins is

Answer 12

exploited in analytical techniques such as immunoblotting.

Question 13

good muscle components diagram on pg 700

Answer 13

kk

Question 14

pg 705 he cycle has four major steps

Answer 14

In step , ATP binds to
myosin and a cle in the myosin molecule opens, disrupting the
actin-myosin interaction so that the bound actin is released. ATP
is then hydrolyzed in step , causing a conformational change in
the protein to a “high-energy” state that moves the myosin head
and changes its orientation in relation to the actin thin filament.
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Myosin then binds weakly to an F-actin subunit closer to the Z
disk than the one just released. As the phosphate product of ATP
hydrolysis is released from myosin in step , another
conformational change occurs in which the myosin cle

Question 15

Because there are many myosin heads in a thick filament, at any
given moment some (probably 1% to 3%) are bound to thin
filaments. This prevents

Answer 15

thick filaments from slipping backward
when an individual myosin head releases the actin subunit to
which it was bound. The thick filament thus actively slides
forward past the adjacent thin filaments. This process,
coordinated among the many sarcomeres in a muscle fiber,
brings about muscle contraction.

Question 16

The interaction between actin and myosin must be regulated so
that

Answer 16

contraction occurs only in response to appropriate signals
from the nervous system. The regulation is mediated by a
complex of two proteins, tropomyosin and troponin (Fig. 5-30).
Tropomyosin binds to the thin filament, blocking the attachment
sites for the myosin head groups. Troponin is a Ca
2+
-binding
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protein. A nerve impulse causes release of Ca
2+
ions from the
sarcoplasmic reticulum. The released Ca
2+ binds to troponin
(another protein-ligand interaction) and causes a conformational
change in the tropomyosin-troponin complexes, exposing the
myosin-binding sites on the thin filaments. Contraction follows good pic on pg 707

Question 17

Protein-ligand interactions achieve a special degree of

Answer 17

spatial
and temporal organization in motor proteins. Muscle contraction
results from choreographed interactions between myosin and
actin, coupled to the hydrolysis of ATP by myosin.

Question 18

Myosin consists of … Myosin molecules are organized into…

Answer 18

-two heavy chains and four light chains,
forming a fibrous coiled coil (tail) domain and a globular (head)
domain
-thick filaments,
which slide past thin filaments composed largely of actin. ATP
hydrolysis in myosin is coupled to a series of conformational
changes in the myosin head, leading to dissociation of myosin
from one F-actin subunit and its eventual reassociation with
another, farther along the thin filament. The myosin thus slides
along the actin filaments

Question 19

Muscle contraction is stimulated by the release of

Answer 19

Ca
2+
from
the sarcoplasmic reticulum. The Ca
2+ binds to the protein
troponin, leading to a conformational change in a troponintropomyosin complex that triggers the cycle of actin-myosin
interactions

Question 20

Protein function often entails

Answer 20

interactions with other
molecules. A protein binds a molecule, known as a ligand, at its
binding site.

Question 21

Myoglobin contains a heme prosthetic group, which binds

Answer 21

oxygen. Heme consists of a single atom of Fe
2+
coordinated
within a porphyrin.

Question 22

Globins are a specialized family of

Answer 22

transport proteins
containing heme; most globins store oxygen.

Question 23

Oxygen binds to myoglobin…

Answer 23

reversibly.

Question 24

Reversible binding of a ligand to a protein can be described
quantitatively by

Answer 24

a dissociation constant Kd. For a monomeric
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protein such as myoglobin, the fraction of binding sites occupied
by a ligand is a hyperbolic function of ligand concentration.

Question 25

Proteins may undergo conformational changes when a ligand
binds, a process called

Answer 25

d induced fit. In a multisubunit protein, the
binding of a ligand to one subunit may affect ligand binding to
other subunits. Ligand binding can be regulated.

Question 26

Hemoglobin transports oxygen in

Answer 26

blood

Question 27

Normal adult hemoglobin has four

Answer 27

heme-containing subunits,
two α and two β, similar in structure to each other and to
myoglobin.

Question 28

Hemoglobin exists in two interchangeable structural states

Answer 28

T
and R. The T state is most stable when oxygen is not bound.
Oxygen binding promotes transition to the R state.

Question 29

Oxygen binding to hemoglobin is both

Answer 29

allosteric and
cooperative. As O2 binds to one binding site, the hemoglobin
undergoes conformational changes that affect the other binding
sites — an example of allosteric behavior

Question 30

Conformational changes between the T and R states, mediated
by subunit-subunit interactions, result in

Answer 30

cooperative binding;
this is described by a sigmoid binding curve and can be analyzed
by a Hill plot

Question 31

Two major models have been proposed to explain the
cooperative binding of ligands to multisubunit proteins:

Answer 31

the
concerted model and the sequential model

Question 32

Hemoglobin also binds H
+
and CO2, resulting in the formation
of

Answer 32

ion pairs that stabilize the T state and lessen the protein’s
affinity for O2 (the Bohr effect).

Question 33

Oxygen binding to hemoglobin is also modulated by 2,3-
bisphosphoglycerate, which binds to and stabilizes

Answer 33

s the T state

Question 34

Sickle cell anemia is a genetic disease caused by

Answer 34

a single amino
acid substitution (Glu
6
to Val
6
) in each β chain of hemoglobin.
The change produces a hydrophobic patch on the surface of the
hemoglobin that causes the molecules to aggregate into bundles
of fibers. This homozygous condition results in serious medical
complications